Molluscs from polar expeditions reveal new details about the ocean

In the early hours of October 30, 1961, a Russian bomber took off and headed north. The plane was heading towards the Novaya Zemlya archipelago, in the Russian part of the Arctic. When the pilot saw the islands far below, he dropped the cargo, a bomb the size of a double-decker bus.

As the pilot accelerated out of range, the bomb slowly descended to the ground under the canopy of a huge parachute. A minute passed, and then the sky was lit up with the harshest light ever created by man.

The bomb, later named Tsar Bomba, is the most powerful nuclear weapon ever detonated. It was the culmination of nuclear testing carried out by the USSR, the USA and several other countries in the years following World War II.

Two years later, in 1963, the nuclear powers agreed to cease atmospheric nuclear testing and the tests were moved underground.

However, after nearly 20 years of detonations – from the first in 1945 until the 1963 treaty – the chemistry of the oceans had changed; it is a change that will last for thousands of years.

For example, American and French detonations in the Pacific Ocean killed thousands of fish and depleted the region’s biodiversity. But the tests also had another consequence. They made it difficult to use carbon-14 dating.

Researcher Christof Pearce from the Department of Geosciences, Arctic Research Center and iClimate at Aarhus University and several of his colleagues tried to find a method to get around these problems. Their work is published in Geochronology.

“We cannot calibrate the age of carbon-14 from fossilized animals or plants found in ocean sediments. Nuclear testing created massive amounts of carbon-14 in the atmosphere, which was slowly absorbed by the ocean. While the atmosphere quickly returned to some sort of equilibrium, it will be hundreds, if not thousands, of years before the ocean can do the same,” Pearce explains.

“That’s why we need material from before the nuclear tests – and that’s where polar expeditions come in. We can use them to find out how much carbon-14 was there before the detonations and adjust the dating.

What is carbon-14 dating?

The Earth’s atmosphere is made up of a number of gases such as oxygen, CO₂ and nitrogen.

When nitrogen atoms enter the upper layers of the atmosphere, they are struck by free neutrons released by cosmic radiation. Nitrogen atoms absorb neutrons, are converted to carbon-14, and emit a proton. The new carbon-14 atoms then bond with oxygen, creating CO₂.

Trees, shrubs and other plants absorb CO₂ from the air during photosynthesis, which means they also absorb carbon-14.

However, the vast majority of CO₂ in the atmosphere is made up of carbon 12. Only a small proportion of CO₂ contains carbon-14, which is radioactive. Plants therefore mainly absorb carbon-12.

When a plant dies, it stops absorbing new carbon, but because carbon-14 is radioactive, it slowly decays and disappears. Carbon-12, on the other hand, does not. Researchers can calculate the age of a plant residue by measuring the amount of carbon-14 remaining relative to carbon-12. We know that the half-life of carbon-14 is 5,700 years and that the natural distribution between carbon-12 and carbon-14 is quite stable.

Plants are eaten by herbivores, which thus absorb the carbon. The herbivores are in turn eaten by the carnivores, who also absorb it. This is why the method can also be used to carbon-15 date animals and humans.

Museums are a treasure

Nuclear testing isn’t the only reason the age of carbon-14 in the oceans has changed. Human CO emissions₂ also changed the balance, but in the opposite direction.

As their name suggests, fossil fuels are made of fossil plant materials and therefore have a high carbon-14 age. CO₂ the emissions therefore had the opposite effect to that of the atomic bombs which created new carbon-14. This is a well-known problem, especially to geologists, archaeologists and other researchers interested in the appearance of the ocean in the past.

For Pearce, sediment samples are one of the most important sources of knowledge about past climate and marine environments. Just like researchers who drill ice cores into the ice sheet to study past climate, Pearce and his colleagues use cores from the seafloor.

The different layers of the cores are full of microfossils and organic matter that can reveal what the oceans looked like in the past.

But to use the knowledge stored in cores extracted from the seabed, researchers need to know when the layers formed. And that’s where carbon-14 comes into play.

“The problem is that we don’t know the natural level of carbon-14 everywhere in the ocean. We don’t have a zero point like we do in the atmosphere. When we try to date a layer of sediment, we are “We often need materials of a known age that predate major human disturbance. I’ve thought for a long time about how we might solve the carbon problem for the ocean in parts of the Arctic,” says Pearce.

“When I was working in Stockholm, I walked past the display cases lining the corridors of the university. They all speak of ancient polar expeditions. That’s when I thought that samples from before the nuclear tests could still exist and which I could examine.”

Samples from Danish and Swedish shipments

Pearce then set about discovering whether any samples from the old expeditions had been preserved. If they were kept, he considered whether he could use them. He quickly discovered that Denmark and Sweden kept samples from their polar expeditions in museum collections.

One of the ancient expeditions he examined left Copenhagen almost 100 years ago. It was called the Godthaab Expedition, and although it is one of the lesser-known expeditions, the researchers who participated in it brought back a lot of valuable knowledge. This knowledge now represents a gold mine for researchers like Pearce.

“The expedition sailed between Greenland and Canada. It measured salinity and water temperature, measured depth, took bottom samples and collected mussels. A colossal effort. Fortunately, the samples and Records from that time are still in the storage rooms of the Copenhagen Zoo Museum. And as luck would have it, we were allowed to take some samples with us. This allowed us to test carbon-14 levels in the ocean before nuclear tests.

The Swedish Museum of Natural History in Stockholm also allowed the research team to take samples from ancient polar expeditions.

A nauseating job

Pearce and his colleagues brought nearly 100 samples back to their lab.

However, only mussels and snails containing soft tissue may be used. And preparing them for analysis was not a task reserved for those with a delicate sense of smell.

“We fished half-rotten mussels and snails out of the old glass bottles. It smelled horrible but we had to take them out and dry them before we could use them. Once the samples were ready, they were taken to the Department of Physics and Astronomy, home to the only laboratory in Denmark capable of carrying out carbon-14 dating.”

Then the researchers waited impatiently for the results, crossing their fingers that the samples were good enough.

A more precise C14 dating

Fortunately, the samples were good, and once the results began, Pearce could see that they would have enough data to more accurately date the materials from Greenland.

“And that’s not all, we now know much more about local variations. The concentration of carbon-14 in the ocean is affected by ocean currents. The lowest values ​​were found around Baffin Bay , between Canada and Greenland, where the influence of the Arctic Ocean is strongest. Low values ​​have also been found in areas where there is a lot of sea ice, which acts as a barrier between the atmosphere and the ocean,” says Pearce.

He explains that the new calculations will make research into past ocean climate more precise. This knowledge is crucial if we want to predict how climate change will affect the oceans in the future.

“In order to calculate what will happen to Arctic waters in the future, we need to know how the ocean has changed over the past few thousand years. And now we have a tool to help us do that more precision,” he explains.

“Also, I think we showed how important it is to preserve the samples in old museum collections. They may not have much value today, but you never know, they might be important in the future, just like the centenary exhibition. -old molluscs suddenly became important to us.”